Piezo electric crystal system



5Pf 29 1931- y J. R. HARRl-soN l 1,824,777

K PIEZO ELECTRIC CRYSTAL SYSTEM Filed oct. 15. 1929v 2 sheets-sheet 1.

' INVgNToR. amai/aow a. fa/:mow

ATTORNEYv SePt- 29, 193i J. R. HARRISON l1,824,777

PIEZO ELECTRIC CRYSTAL SYSTEM Filed Oct. 1 5. 1929 2 Sheets-Sheet 2 www IN VEN TOR.

lil

` invention as set forth more clearl Patented Sept. 29, 1931 UNIT-"ED STATES PATENT OFFICE JAHISON R. HARRISON, OF PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO WIRED RADIO,

INC., OF NEW YORK, N. Y., A CORPORATION OF DELAWARE PmZO ELECTRIC CRYSTAL SYSTEM Application illed October 15, 1929. 4Seriall lo. 399,738.

My invention relates broadly to4 piezo electric crystal systems and more particularly to methods of exciting such crystal elements into oscillation at harmonics of their fundamental shear frequency.

One of the principal objects of my invention comprises providing a novel mounting means for, and system of applying electric charges, fields, or potentials to crystal elements whereby a crystal may be readily and easily excited to vibrate at the even harmonics of its fundamental frequency.

Another object contemplated by my invention consists in providing a more efficient method than that hitherto known for excit` ing a crystal to vibrate at the odd harmonics of its fundamental frequency.

Other and further objects of my invention .reside in the construction and arrangement of mounting means for piezo electric crystal elements and the application of potentials or fields thereto, to cause such elements to vibrate at an desired harmonics of their fundamental s ear frequency according to my in the specification hereinafter following y reference to the accompan ing drawings in which Fig. 1 illustrates dyiagrammatically the relation of a 30 degree cut plate quartz crystal element to the crystal mass from which such element is cut;

Fig. 2 is an end view with reference to its axes of a 30 degree cut plate quartz crystal showing in dotted lines the mode of vlbration' Fig. 3 illustrates diagrammatically a mounting method and associated apparatus and circuits for exciting a 30 degree cut plate crystal to vibrate at the fundamental shear frequency and the odd harmonics thereof;

Fig. 4 illustrates diagrammatically a.30 degree cut plate crystal showing in dotted lines the shear vibration when the same is excited to vibrate at the third harmonic by means of the circuit shown in Fig. 3;

Fig. 5 is a diagrammatic representation of the crystal element of Figs. 3 or 4 when excited in accordance with my invention to vibrate at the second harmonic of the funda mental shear frequency, showing the polarities applied and indicating in dotted lines the modes of vibration;

Fig. 6 illustrates diagrammatically the method of mounting and associated circuits of my invention to excite a crystal element to vibrate at the second harmonic of its fundamental shear frequency;

Fig. 7 illustrates diagrammatically a crystal showin the modes of vibration and distributed po arities of such an element vibrating in accordance with my invention at the fourth harmonic of the fundamental shear frequency;

Fig. 8 illustrates diagrammatically the method of applying the potentials in accordance with my invention to accomplish the result diagrammatically indicated in Fig. 7.

It has hitherto been well known from the piezo electric theory developed by Cady that 3Q degree cut plate quartz crystals, as plates cut from quartz crystal parallel to one of the hexagonal faces-of the crystal are called in the literature relating thereto, would, when used as oscillators at the high frequency mode of vibration, execute a shear vibration. The theory of such shear vibration is discussed by W. G. Cady in volume 29, page 617. The manner of cutting such plates from a crystal mass is illustrated in Fig. 1. The dotted lines 1 and 2 indicate the planes arallel to X axis and erpendicular to the axis on which a 30 egree plate may be cut. The shear vibration of such a Dlate is illustrated by dotted lines in Fig. 2, which shows an end view of a quartz plate. cut from a crystalas indicated in Fig. 1. In Fig. 2, X indicates the direction of the electrical axis; the optical axis is perpendicular to the paper on which the drawing is made, and the Y axis is parallel to the thickness of the plate and perpendicular to both the electrical and the optical ax'es. For this type of shear vibration, the thickness of the plate corresponds to one-half of the wave length of the shear vibration frequency. The electromagnetic wave length in meters corresponding to the frequency of the shear vibration is approximately equal to 140 times the thickness of the plate in millimeters.

It has been hitherto known that it was easily possible to excite a 30 degree c'ut plate crystal into oscillation at its odd harmonics,

but believed quite impossible to cause it to oscillate at its even harmonics. The present disclosure describes a crystal mounting and associated apparatus and circuits whereby the even harmonics of a crystal may be readily excited or the odd harmonics more efliciently produced by using a new type of crystal mounting and associated circuits whereby the potentials or fields of the properly related sign are applied to the crystal. Foriexample, a 30 degree cut crystal approximately 6.5 mms. thick has the shear oscillation frequency length of approximately 900 meters. In practice, it has been found possible to excite the fundamental or first overtone of 900 meters, the third overtone of 300 meters, the fifth overtone of 180 meters, etc. It has hitherto been found, however, quite impossible to excite the even overtones; that is, for example, the second overtone of 450 meters, the fourth overtone of 225 meters, the sixth overtone of v150 meters, etc. It is my main purpose here to describe a new type of crystal mounting v that applies the electric fields or potentials in such manner with respect to the relation of the polarities thereof that the even harmonics can be quite easily excitedy and made use of in piezo electric oscillator circuits, as resonators for frequency standards, and as coupling devices. The method of excitation can also be harmonics as will hereinafter be pointed out. Figa 3 indicates the usual circuit ,for exciting a degree cut crystal to vibrate in the fundamental shear vibration or y/at the odd harmonics thereof. The circuit comprises the usual thermionic tube 3, the cathode of which is energized by a source of current 6, and the anode potential applied j from source 9, bridged as usual by condenser l0. A tuning circuit 7 is provided intlieanode circuit, to gether with the tuned' output circuit 8. 'Ihe grid circuit includes a resistance 4, and biasing battery 5 as usual. Plates 12gand 13, connected to the grid through conductor 14, and the cathode through conductor 18", respectively, serve as electrodes for applying exciting potentials tothe crystal 11.` It will be noted that, when the upper surface of the crystal has a positive charge induced thereon, the lower surface has induced thereon a nega- 5455 tive charge, and vice versa. Sustained oscillations are produced in such a circuit through feedback provided by the interlelectrode capacity of the tube, the frequency being determined by the crystal and the adjustment of 30 circuit 7.

Fig. 4 illustrates t'h crystal 11 vibrating at the third harmonic of the shear vibration. As before, when the upper surface of the crystal is positive, the lower surface E6 is negative, and vice versa; thus the same corresponding to an electromagnetic wave.

used for more efficient production of the odd "method of excitation as shown inFig. 3 for the fundamental or first overtone of the shear vibration. The dotted lines 15 and 16, Fig. 4, show the single crystal plate divided into three fictitious crystal pl'ates, all exciting a simple shear vibration like the same crystal at the fundamental mode as shown in Fig. 3. The middle fictitious crystal is, in effect, reversed with respect to the others so that the polarity of the charges on its fictitious faces is reversed with respect to the others. The polarity of the chargesaon the faces of all of the fictitious crystals is shown as taken for a given instant in time when a positive charge is induced on the upper surface and a negative charge on the lower surface of the crystal. Y

The condition just described exists for all the odd overtones of the shear vibration. In other words, a positive charge on the-upper surface and a negative charge on the lower surface, or vice versa, will always be present when a crystal is vibrating at the fundamental or at any of the odd harmonics thereof This can be shown from theoretical considerations and from actual experimental tests. Fig. 5 illustrates diagrammatically yhow the crystal plate l1 would look when vibrating at the second harmonic of the shear vibration. Here. the crystal plate is considered as being niadeup of two fictitious plates, the dividing line being indicated by the dotted line 17.' Each fictitious plate is one-half as thick as the whole crystal and each vibrates in 'a simple shear like the whole plat-e when vibrating at the fundamental mode plained with reference to Fig. 3. On examining the charges on the surfaces of the fictitious plates in Fig. 5 as we did in Fig. 4, we find that the charges on the two outer surfaces of the crystal for this mode of vibration are always of the same polarity.

Both are, at any given time, either positive or negative. The plate could not, Obviously, therefore, be excited into oscillation with the circuit and crystal mounting connected as shown in Fig, 3. It would not be possible with such a circuit to provide the necessary like polarities .on the two out'er surfaces of the crystal. In the same way, it can be shown by experimental tests that the charges which appear on the electrodes at the outer surface of the plates are always of the same polarity for all of the even hamonics of the shear vibration. I have found it possible to excite the even harmonics of this shear vibration by utilizing the mounting and associated circuit illustrated diagrammatically as exin Fig. 6. In this circuit the usual thermionic tube 3 is provided With the usual energy sources 6 and 9, tuning circuits 7 and 8, grid resistance 4, and biasing battery 5. The crystal mounting, however, in this case consists of three electrodes, two plate electrodes 22 and 24, each connected to the grid of the tube 3, and in addition a third electrode 23, which is frame-shaped, and about one-third as thick as the crystal in the direction of the Y axis thereof, extending around the outer sides of the crystal and making contact therewith. This electrode is connected through conductor 18 with the cathode of the tube 3 and it is symmetrically disposed with respect to the crystal and the other electrodes 22 and 24. The other dimensions of the frame-shaped electrode are not critical but they should be made as small as possible so that the capacity effects between it and the other electrodes are reduced to a minimum. All of the electrodes are so designed that they enclose the crystal without vdamping it critically and are held in their relative positions and in position with respect to the crystal bymeans of insulators. Further explanations of such mounting is not deemed necessary, as they are obvious to anyone familiar with the art.

QWhen the crystal shown in Fig. 6 is excited to" vibrate on the second harmonic of the shear vibration, a positive charge is at any given time induced on both' electrodes 22 and 24 and a ne ative charge is induced on the rameshape electrode 23 and vice versa, to permit the crystal to vibrate in accordance with my invention at' the even harmonics of the shear vibration. When the tuning circuit 7 is tuned approximately to the frequency of the second harmonic of the crystal, the conditions for oscillation in. the circuitare satisfied. Electrode 23 can, of course, be connected to the grid instead of the filament and electrodes 22 and 24 in which case are connected to theifilament.

In Fig. 7 there is shown crystal 11 as it would appear vibrating at the fourth harmonic. In this case the crystal is equivalent to four fictitious crystals each one-fourth as thick asA the crystal itself, and the dividing lines are indicated by the dottedv lines 25, 26, and 27. The charges on the real and fictitious plate faces are in this case as indicated in the figure. crystal mounting as shown in Fig. 8 is used, consisting of five electrodes, two of these, 22 and 24, are flat plates covering the broad surfaces of the crystal or faces thereof, and '31, 23 and 32 are frame-shaped' electrodes each approximately one-eighth as thick as the crystal and spaced equi'distant apart with respect to themselves and the outer edges of the crystal. An examination of the charges yon the crystal as shown in Fig. 7 indicates that electrodes 22, 24, and 23 in Fig. 8 would To excite this mode of vibration a induce charges of the same polarity, while.

electrodes 3l and 32 would have charges 0f the same polarity but opposite to that of 22, 24 and 23. In order to accomplish this, electrodes 22, 24, and 23 are connected together and to the grid of the vacuum tube oscillator circuit shown in Fig. 6 and electrodes 3l and 32 are connected together and to the filament of tube 3 in Fig. 6 or vice versa.

n In a similar manner the sixth harmonic ot the crystal can be excited. A mounting with two fiat electrodes and five frame-shaped electrodes is used. Similar mountings are constructed and used for exciting all of the even harmonics of the crystal. In exciting the higher harmonics more and more of the frame-shaped electrodes are used. It is not necessary, however, for any given higher mode to use all of the electrodes that could be used as seen from theoretical considerations, but in each case all ofthe frame-shaped electrodes that are used must be properly placed with respect to the crystal; that is, at thev boundary of two adjacent fictitious plates. Also, these frames must not be too thick; that is, they must not overlap more than two of the fictitious crystals for a given mode.

Having thus completely described my invention, what I claim is:

1. In a piezo electric crystal system, a source of electrical potentials, a piezo electric crystal, contact plates, the surfaces of which are parallel to the plane of the X axis and perpendicular to the plane of the Y axis of said crystal and in contact with the faces of said crystal, a plurality of electrodes each adapted to make contact with all of the sides of said crystal in a plane parallel to the X axis and perpendicular to the Y axis thereof, and means for applying from said source to said plates and said electrodes potentials of predetermined related sign.

2. In a piezo electric crystal system, a source of electrical potentials, a piezo electric crystal element cut to have two faces, the planes of which are parallel to the X axis and perpendicular to the Y axis of said element, and means for utilizing potentials from said source to produce fields of like sign on both of the faces of said element and of like sign simultaneously on all of the sides of said element, the latter fields being of different sign from the fields produced on the faces of saidcrystals.

3. In a piezo electric crystal system, a source of electrical potentials, a piezo electric crystal element cut to have two faces, the planes of which are parallel to the X axis and perpendicular to the Y axis of said element, and means for utilizing potentials from said source to induce charges of like sign on both of the faces of said element and charges of like but opposite polarity to that induced on the faces of said element on all of the sides of said element simultaneously.

4. In a piezo elect-rie crystal system, a source of electrical potentials, a piezo electric crystal element cut to have two faces, the planes of which are parallel to the X axis and Y u perpendicular to the Y axis of said element and means for utilizing electrical potentials ssfrorn said source, co-actinp,lr with said element 1l) to produce electrical oscillations of definite frequencies, said means including means for establishing Contact between said source of potentials and said element through the two faces and all of the sides of said element simultaneously.

, 5. In a piezo electric crystal system, a `source of electrical potentials, a piezo electric crystal cut to have two faces, the planes of which are parallel to the X axis and perpendicularv to the Y axis of said crystal, and means for applying from said source, at any given time, to ylooth of the faces of said crystal, potentials of the saine polarity, and to" all of the sides of said crystal, potentials of the saine polarity but of opposite sign to the potentials applied to the faces thereof.

6. In a piezo electric crystal system, a source of eeletrical potentials, apiezo electric crystal element` a pair of plates, the surfaces of each of which are parallel to the plane of the X axis andperpendicular to the plane of the Y axis ofsaid element and iny contact with the respective faces of said element, an electrode in contact with all if the sides of said element in a plane parallel to the X axis' and perpendicular to the Y axis of said element, means for applying from said source, electrical potentials of like sign to said plates and means for applying from said source a potential of unlike sign to that on said plates to said electrode.

7. In a piezo electric crystal system, a source of electrical potentials, a piezo electric crystal cut to have two faces', the planes of I which are parallel to the X axis and erpendicular to the Y axis of said crysta plate electrodes in contact with each of the faces of said crystal, a plurality of frame shaped electrodes, each of which niakes contact with 5@ all of the sides of said crystal in a plane pari allel to the X axis and perpendicular to the Y axis thereof, and means for applying from said source to all of said electrodes potentials 'of predetermined related sign.

.a5 JAMISON R. HARRISON. 

